Treatment of MDCK II cells with the lipophilic photosensitizer tetra(3-hydr
oxyphenyl)porphyrin and light was found to induce a rapid apoptotic respons
e in a large fraction of the cells. Furthermore, the distribution of apopto
tic cells in microcolonies of eight cells was found to be different from th
e binomial distribution, indicating that the cells are not inactivated inde
pendently, but that a bystander effect is involved in cell killing by photo
dynamic treatment, The observation of a bystander effect disagrees with the
common view that cells are inactivated only by direct damage and indicates
that communication between cells in a colony plays a role in photosensitiz
ed induction of apoptosis. The degree of bystander effect was higher for ce
lls dying by necrosis than for cell dying by apoptosis. (C) 2000 Elsevier S
cience B.V. All rights reserved.